Quantities of liquid medium-containing materials are often stored so as to be ready for use when needed. As used herein, the term "liquid medium-containing materials" are materials which include a liquid, for example, a major amount of liquid, and are flowable. Such materials may include solids and/or gases. In many instances, the rate at which such materials are supplied to a storage system is independent of the rate at which such materials are removed from the storage system for use. Such materials can be supplied to the storage system from production facilities directly linked to the storage system and/or by periodical deliveries from remote production facilities. The storage system advantageously maintains an adequate supply of the liquid medium-containing material to meet use demands without becoming overfilled. Further, where a production facility is directly linked to the storage system, the operation of such facility is beneficially not adversely impacted by the storage system. This is illustrated by considering a reverse osmosis module as such a production facility.
Reverse osmosis is a commonly used technique for removing impurities from drinking water. In a conventional reverse osmosis system, a reverse osmosis module is coupled to a source of feed water under pressure, and the module provides filtered product water having a reduced level of impurities as well as brine or waste water which is commonly dumped to drain. Reverse osmosis systems of this type are adapted for domestic household use as well as for other purposes.
A reverse osmosis module typically produces product water at a relatively slow rate. The demand for product water, although intermittent, is normally at a rate much greater than the instantaneous production capability of product water by the reverse osmosis module. Consequently, a reverse osmosis system typically includes a storage tank for storing product water it is produced so that the intermittent, but higher flow rate, demand can be met from the stored water in the tank. In order that this stored water can be provided under pressure to the water tap, it is common practice to employ some kind of accumulator or pressurized storage tank for storing of the product water.
A reverse osmosis module contains a reverse osmosis membrane for removing the impurities. A characteristic of these membranes is that their efficiency is greater at higher differential pressures across the membranes. For example, at higher differential pressures product water is produced at a faster rate. Also, a higher differential pressure across the membrane reduces the quantity of brine and therefore the water loss, and this is of particular importance in arid regions or regions where water shortages may exist.
One problem with an accumulator or pressurized storage tank for the product water is that they provide a back pressure on the membrane, and this reduces the differential pressure across the membrane with consequent slower production of product water and increased brine production. Thus, the pressurized storage system adversely impacts the performance of the reverse osmosis production facility. In addition, the means utilized for pressurization of the storage tank, whether in the form of an air chamber or spring biased members, can be expected to require additional space, and this is also a disadvantage especially for household systems which are commonly installed under the kitchen sink. Cost is also increased because the pressurized container must meet pressure vessel design standards.